Field of the Invention and Related Art Statement
The present invention relates to an apparatus for actively controlling a steer angle of front wheels of a vehicle, and more particularly to an apparatus for actively controlling a steer angle of front wheels of a vehicle in accordance with a steering wheel angle and a running velocity of a vehicle.
There has been proposed an apparatus for controlling a steer angle of front wheels of a vehicle, in which, in addition to a usual control of the front wheels in proportion to a steering wheel angle, the steer angle of the front wheels is auxiliarily controlled in an active manner in accordance with the steering wheel angle, an angular velocity of the steering wheel angle and running velocity of the vehicle. There has been also proposed an apparatus for auxiliarily controlling both the front wheels and the rear wheels in an active manner. The latter apparatus is utilized in a so-called four wheel steering (4 WS) vehicle. For instance, in Japanese Patent Publication Kokai Sho No. 60-161,266, the above mentioned steer angle controlling apparatus is described.
In the known apparatus for actively, controlling the steer angle of front wheels, an active control amount for auxiliarily or actively controlling the steer angle of the front wheels is calculated by applying the steering wheel angle, a differential thereof and running velocity to a predetermined equation including a proportional term and a differential term, and the steer angle of the front wheels is additionally increased by the thus calculated active control amount. This improves the stability and handling properties of the vehicle and creates a new balance of the vehicle. For instance, when the steer angle of the front wheels is increased by the active control amount, upon a turn of the vehicle, a turning characteristic of the vehicle can be improved due to an increase in a yaw rate, i.e. an angular velocity of a yawing movement of a body of the vehicle.
In the known apparatus for controlling the steer angle of the front wheels in the active manner, the active control amount is derived by effecting a calculation based on a fixed equation. Therefore, the active control in a case of changing a steering direction from a straight-ahead direction and the active control in a case of changing the steering direction from a turning movement are carried out in the same manner, because the active control amount is calculated by the same equation without taking into account the driving conditions. Therefore, the variation of the cornering power could not be compensated for.
FIG. 1 shows a relation between a cornering force C.sub.F generated in tires and a slip angle .beta.. In a range I in which the slip angle .beta. is relatively small, there is a substantially linear relationship between the slip angle and the cornering force, but in a range II in which the slip angle is relatively large, a non-linear relationship appears due to the non-linearity of tires. In this manner, the cornering power C.sub.P is not sufficiently increased in the range II, in which a high lateral acceleration (high lateral G) turning is performed. Therefore, when various coefficients in the equation for calculating the active control amount are determined in accordance with the range I in which the linear relationship between the slip angle .beta. and the cornering force C.sub.F (cornering power C.sub.P) is obtained, these coefficients are not suitable for the non-linear range II which happens when the high lateral G turning and the cornering power of the tires is decreased. In this manner, in the known active control apparatus, the behavior of the vehicle varies greatly, dependent on various driving conditions of the vehicle. In this manner, the known apparatus for controlling the steer angle of the front wheels in the active manner has a drawback that it is difficult to attain a sufficient response, although the stability and handling properties in the above mentioned range I can be improved, so that the head-turning property in the high lateral acceleration turning such as the high speed lane change could not be improved.